The present invention relates generally to milling machines, and more particularly to an automated quill drive assembly that is adapted for retrofitted use on a conventional milling machine to drive Z-axis movement of the quill in response to control from a conventional computer numerical control ("CNC") controller or the like.
A conventional milling machine, such as a knee mill, includes a table on which a work piece can be supported, and a quill that supports a tool for movement toward and away from the table for milling the work piece. Typically, the table is adjustable within a horizontal plane from side-to-side and from front-to-back relative to the quill. Lateral side-to-side adjustment is considered X-axis adjustment, and front-to-back adjustment is referenced as Y-axis adjustment. As such, the position of a work piece can be changed in two planes, and the quill moves vertically along a Z-axis such that three-axis adjustment of the tool relative to the work piece is possible.
Although many milling machines are manual, in the sense that adjustment of the table and movement of the quill is manually achieved, improvements in the art have resulted in fully automatic machines in which X-axis, Y-axis and Z-axis movement of the table and/or quill are computer controlled and carried out by stepper or servo motors. These automatic machines provide many advantages over manual machines, especially with respect to the reproducibility of milling operations performed with such milling machines. However, automatic machines lack the versatility of manual designs, and cost significantly more to purchase and use.
Automated drive assemblies are known that can be retrofitted on an existing milling machine to automate the X-axis and Y-axis movement of the table and the Z-axis movement of the quill. Typically, a CNC controller is installed on the machine at the same time as one or more of these drive assemblies such that the drive assemblies are capable of being operated automatically subsequent to installation.
A conventional Z-axis or quill drive assembly broadly includes a housing supported on the quill head of the machine, a stepper or servo motor supported on the housing and operatively connected to a conventional CNC controller, and a transmission that is supported on the housing and connected between the motor and the quill to transmit the motor drive to the quill so that when the motor is operated, the quill is moved between the retracted and extended positions. The housing is secured to the head at three points, one of which is defined by a lower casting lug of the quill head normally used to support a depth stop screw. The second and third points of attachment of the housing are defined by a pair of vertically spaced tramming bolts provided along the right front side of the quill head.
In the conventional automated quill drive assembly, the position of the quill is continuously monitored by providing feedback from the motor to the CNC controller indicative of the position or operation of the motor. In addition, limit switches are sometimes employed to provide a positive signal when the quill is moved to either the fully retracted or fully extended limit position. In this way, the CNC controller is able to control operation of the motor to accurately position and reposition the quill along the Z-axis during milling operations.
Although conventional automated quill drive assemblies do allow a manual milling machine to be retrofitted for automatic operation, further improvements would be advantageous. For example, because the housing of a conventional assembly is mounted to two of the tramming bolts on the quill head, the housing must be loosened in order to tram the head. Such added complexity makes tramming of the quill head difficult and time consuming.
Further, although three mounting points are provided for the housing of a conventional quill drive assembly, two of the points are in line with one another vertically, and the third point is spaced laterally from the other two by only 1 to 2 inches. As a result, the motor can twist or wind up when milling a heavy cut, reducing the accuracy of the machine, and increasing wear on the quill.
Another feature of the conventional quill drive assembly subject to improvement is the source of feedback provided to the CNC controller which allows the position of the quill to be continuously monitored and controlled during automatic operation. Because the conventional assembly monitors quill movement indirectly by sensing motor movement or operation, the CNC controller can only monitor the quill position when the quill is operated by the motor, or when manual quill movement is transmitted back to the motor. As such, manual operation of the quill is either frustrated or must be performed without the benefit of feedback to the CNC controller. When manual operation is performed without such feedback, depth control and repeatability become difficult to achieve.